In case this does anything for someone facing a similar problem:
My H700 headset (fantastic, by the way) would no longer field calls from my Treo 700p. It paired just fine, but it wouldn't accept the audio. Everything I tried (soft, warm, hard resets) did nothing. Eventually I tried using the headset on another cellphone and it worked (great, I thought -- the treo's busted). But then I repaired with my treo and it was fine. I think the headset was confused.

Lesson: if your H700 stops accepting your audio, link it to another phone and then back to yours. You'll be good to go.